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Probing complex low‐dimensional solids with scanning probe microscopes: From
charge density waves to high‐temperature superconductivity
Jie Liu, Jin‐Lin Huang, and Charles M. Lieber
Citation: Journal of Vacuum Science & Technology B 14, 1064 (1996); doi: 10.1116/1.588401
View online: http://dx.doi.org/10.1116/1.588401
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Probing complex low-dimensional solids with scanning probe microscopes:
From charge density waves to high-temperature superconductivity
Jie Liu, Jin-Lin Huang, and Charles M. Lieber
Department of Chemistry and Division of Applied Sciences, Harvard University, Cambridge,
Massachusetts 02138
~Received 24 July 1995; accepted 1 December 1995!
Scanning probe microscopy ~SPM! studies of low-dimensional solids have yielded significant
advances in the understanding of collective phenomena in these complex materials. In this article,
SPM studies from the authors’ laboratory of charge density waves ~CDWs! in metal dichalcogenide
materials and of high-temperature copper–oxide superconductors ~HTSCs! are reviewed.
Temperature-dependent SPM studies of the structural properties of CDW phases in metal-doped
tantalum disulfide ~TaS2! and tantalum diselenide ~TaSe2! have directly illuminated the problem of
weak and strong CDW pinning. These studies have also led to the unexpected discovery of a hexatic
CDW phase in niobium-doped TaS2 . SPMs have also been used to probe the structural and
electronic properties of HTSCs. SPM studies of metal-doped Bi2Sr2CaCu2O8 ~BSCCO!
superconductors have been used to elucidate the complex crystal chemistry of this system.
Comparisons of these data with electron diffraction measurements also highlight the importance of
local crystallography in these complex materials. In addition, SPM studies have been used to
elucidate the interaction of magnetic flux lines with surface defects, thus enabling a quantitative
assessment of pinning by surface roughness. Finally, etching has been studied by in situ SPM and
shown to be a promising means for controlling the properties of HTSC surfaces. © 1996 American
Vacuum Society.
I. INTRODUCTION
Understanding the relationships between structure, electronic properties, phase transitions, and observable properties
in materials represents an important goal of condensed matter research.1,2 Approaches to achieving this goal necessarily
involve elucidating how material properties are determined
by the arrangement of atoms and dopants on the atomic
scale. In principle, this information is readily available from
conventional diffraction and spectroscopy measurements
when solids exhibit highly ordered, crystalline structures.
However, some of the most fascinating solids known today,
such as the high-temperature superconductors ~HTSCs!, are
not well ordered. These complex materials exhibit structural
complexities ~e.g., substitutional disorder and/or incommensurate modulations! that conventional diffraction measurements cannot resolve. Experimental techniques that can
probe material properties directly on the nanometer scale
should thus afford researchers a unique opportunity to unravel these problems.
The ability of scanning probe microscopes ~SPMs! to
probe surface structure and electronic states on the atomic
scale is now well recognized.3 SPMs can also provide information essential to understanding bulk properties in many
classes of materials.2,4 –7 In particular, solids that possess a
large anisotropy in bonding—for example, strong covalent
bonding in two-dimensional layers with weak noncovalent
bonds holding these layers together—cleave preferentially
along planes defined by the weak covalent bonds within the
solid. The coordination of atoms at the surface cleavage
plane in such anisotropic or low-dimensional solids is similar
to that in the bulk, because the covalent bonding is un1064
J. Vac. Sci. Technol. B 14(2), Mar/Apr 1996
changed. Hence, these cleaved surfaces usually do not reconstruct and their structure and electronic properties are thus
representative of the bulk solid.
Highly anisotropic solids are amenable to SPM studies
but, more importantly, these materials often exhibit scientifically fascinating structural and/or electronic phase transitions
that represent one of the focal points of condensed matter
research today. For example, a number of one- and twodimensional transition metal chalcogenide materials are
known to exhibit complex charge density wave ~CDW!
phases and, furthermore, it well known that the layered structure of the copper oxide materials is essential for hightemperature superconductivity. In this brief review, we will
focus on our previous SPM studies of low-dimensional solids that have provided new information addressing important
questions about CDWs and HTSCs. The implications of
these results and ongoing studies will be discussed.
II. EXPERIMENT
An important issue in SPM investigations of lowdimensional solids is the preparation of suitable samples for
analysis since most materials are not available commercially
in single-crystal form. In general, our studies have focused
on high-quality single crystals that are prepared and characterized by conventional techniques; these methods are briefly
described below and have been discussed in more detail
previously.8 –12
Single crystals of transition metal dichalcogenides
~1T–TaS2 and 1T–TaSe2! and metal-doped transition metal
dichalcogenides ~1T–NbxTa12xS2 and 1T–Tix Ta12x Se2!
were grown using iodine-vapor transport from prereacted
0734-211X/96/14(2)/1064/6/$10.00
©1996 American Vacuum Society
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Liu, Huang, and Lieber: Probing complex low-dimensional solids
powders.8 –10 The prereacted powders and iodine were sealed
in quartz tubes and placed in a two-zone furnace with the
prereacted powders in the hotter zone. High-quality crystals
were obtained after a three-week transport period using a
;70 °C temperature gradient. The metastable 1T crystal
polytype is obtained by quenching the reaction tubes from
their growth temperature. In all cases the crystal structure
and composition was verified prior to SPM studies. In addition, the CDW phase transition temperatures were determined by temperature-dependent resistivity measurements.
Single
crystals
of
Bi2Sr2CaCu2O8~BSCCO-2212!,
Bi2Sr2CuO6~BSCCO-2201!, and Pb-doped BSCCO were
grown from CuO-rich and/or Bi2O3-rich melts in MgO
crucibles.11,12 MgO crucibles are used by many research
groups since they do not react extensively with the liquid
phase and yield high-quality crystals with sharp transitions at
T c '90 K.
In general, our SPM measurements have been made on
cleaved single-crystal surfaces of the metal dichalcogenides
and BSCCO superconductors. These materials can be
cleaved readily using tape in air or ultrahigh vacuum ~UHV!.
Several different instruments have been used to acquire the
data discussed below. First, a number of the scanning tunneling microscopy ~STM! studies were carried out in a N2 filled
glove box ~<1 ppm H2O and O2! using commercial ~Nanoscope, Digital Instrument, Inc.! and home-built electronics
that controlled a microscope equipped with a variable temperature sample stage. More recent STM studies have been
carried out using a low-temperature UHV microscope that
can be operated at temperatures between 4.2 and 300 K.
Finally, atomic force microscopy ~AFM! studies have been
carried out in air and in solution using a commercial instrument ~Nanoscope III, Digital Instruments, Inc.! equipped
with a fluid cell.
III. RESULTS AND DISCUSSION
A. Charge density waves
Prior to the discovery of STM, the structural properties of
CDWs had been studies extensively using electron and x-ray
diffraction.13 While diffraction studies provided considerable
insight into the basic structures of the CDWs in TaS2 and
TaSe2 , they were unable to elucidate clearly the structures of
several of the complex incommensurate phases existing in
TaS2 and the effects of metal doping in these materials.2,5
This situation changed significantly, however, following the
demonstration by Coleman and co-workers that the charge
modulation of a CDW could be viewed directly in real
space.14 The ability of STM to determine CDW and atomic
lattice positions simultaneously was subsequently exploited
by our group2,5,7–10,15,16 and others4,14,17,18 to resolve complicated structural details of incommensurate CDW phases in
TaS2 , and to catalog the structures of CDW phases in a variety of transition metal dichalcogenide materials.4
Another general problem that STM has been especially
useful in probing is how CDWs interact with metal impurities substitutionally doped in the atomic lattice. Understanding the nature of this interaction, called pinning, is essential
1065
FIG. 1. STM images of the Nbx Ta12x S2 crystals recorded at 380 ~x50!, 340
~x50.04!, and 315 K ~x50.07!. Delaunay triangulations are shown below
each of the corresponding STM images. Topological defects in the triangulated images are highlighted with shading. Reproduced from Ref. 7.
to understanding the static and dynamic properties of the
CDW state. In general, pinning can be defined as strong or
weak. In strong pinning, the impurity potential dominates the
CDW elastic energy and pins the phases of the CDW at each
impurity site. In weak pinning, the CDW breaks up into constant phase regions that are pinned collectively by
impurities.7 Model investigations of weak and strong pinning
by Nb and Ti impurities in Nbx Ta12x S2 and Tix Ta12x Se2 are
discussed below.
Figure 1 shows STM images of Nbx Ta12x S2 single crystals with values of x ranging from 0 to 0.07; the images were
recorded at temperatures of 380 ~x50!, 340 ~x50.04!, and
315 K ~x50.07! in a glove box. These temperatures were
chosen to ensure that the samples were in the incommensurate state where the CDW interacts only weakly with the
underlying lattice.10 Pure TaS2 exhibits a regular hexagonal
CDW lattice that is characteristic of the known incommensurate state in this material. As Nb atoms are substituted for
Ta in the atomic lattice, the CDW lattice becomes less well
ordered. The disorder in the CDW lattice appears to be due
to the formation of dislocations and other topological defects. It is important to note, however, that the disorder does
not appear to destroy the orientational order of the CDW
lattice.10 The presence of relatively long-range orientational
order can be seen qualitatively by sighting down the rows of
the CDW lattice: this shows that the average direction is
defined regardless of the Nb impurity concentration.
To examine in more detail the topological defects and
order of a lattice, it is instructive to carry out a Delaunay
triangulation of the lattice sites.16 This image analysis
method, which is of general utility for any type of lattice, has
been applied previously by several groups.19 The triangulation uniquely defines the nearest neighbors and thus the coordination number of each CDW maxima; in the triangulation, nearest neighbors are connected by ‘‘bonds’’ with
vertices corresponding to the CDW maxima. Fully coordinated lattice sites have six bonds, while topological defects
containing fewer or greater than six bonds are highlighted by
JVST B - Microelectronics and Nanometer Structures
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1066
FIG. 2. The 13.5313.5 nm2 STM images of TaSe2 ~left!, Ti0.02Ta0.98Se2
~middle!, and Ti0.04Ta0.96Se2 ~right! samples recorded at room temperature.
The bright yellow features in these images correspond to the CDW maxima.
Reproduced from Ref. 9.
shading. The triangulated images are shown below their corresponding STM images in Fig. 1. These data demonstrate
clearly that the pure 1T–TaS2 CDW lattice is defect free, and
that the number of topological defects increases with increasing Nb impurity concentration. Significantly, analysis of the
average topological defect spacing shows that it is much
greater than the average spacing between Nb impurities, and
thus this analysis unambiguously shows that the pinning by
Nb atoms is a weak or collective effect.16
It is also possible to further characterize the order in this
system by calculating the translational, G T ~r!, and orientational, G 6~r!, correlation functions.7,16 Analyses of G T ~r! and
G 6~r! as a function of the Nb impurity concentration show
that ~1! for x50 both G T ~r! and G 6~r! exhibit long-range
order, ~2! for 0,3<0.04 G T ~r! decays exponentially ~i.e.,
shows short-range order! and G 6~r! exhibits long-range order, and ~3! for x>0.07 G T ~r! and G 6~r! decay exponentially. The existence of long-range orientational order but
short-range translational order is an important observation
because it strongly suggests the existence of a hexatic glass
state in these doped materials. Hence, these studies were able
to show that the CDW lattice evolves from a crystalline state
in the pure solid through a hexatic glass state to a liquidlike
amorphous state as the impurity concentration increases.
A very different impurity pinning effect has been observed in our STM studies of Ti-doped TaSe2 .9 The CDW
state in 1T–TaSe2 is commensurate at all experimentally accessible temperatures. In the commensurate CDW state, each
CDW maximum is located on the same symmetric Ta atom
site; that is the CDW is effectively pinned to the underlying
atomic lattice. The driving force for the commensurate state
is an electrostatic interaction between the Ta ions and the
CDW. In the incommensurate state, this electrostatic pinning
term is zero. The strong interaction between the CDW and
atomic lattice in the commensurate state suggests that the
effect of metal impurities should be quite distinct from that
observed in the Nbx Ta12x S2 materials. STM images recorded
on a series of Tix Ta12x Se2 single crystals with x50, 0.02 and
0.04 support this idea ~see Fig. 2!. The CDW lattices observed in samples containing Ti impurities exhibit a regular
hexagonal structure that is similar to the pure sample, and are
thus very different than observed in the Nbx Ta12x S2 materials. The images of the Ti-doped materials also exhibit localized regions where the CDW amplitude is suppressed relative to the surrounding CDW maxima. In addition, we have
FIG. 3. STM images of ~A! Bi2Sr2CaCu2O8 , ~B! Pb0.3Bi1.7Sr2CaCu2O8 , and
~C! Pb0.5Bi1.5Sr2CaCu2O8 . The underlying a–b cell axes are indicated in
~A!. The white bar corresponds to 5 nm in these images. Reproduced from
Ref. 11.
shown that the density of these localized defects increases
linearly with the Ti impurity concentration. These data and
the highly localized nature of the CDW defects represent
solid evidence for strong pinning. Hence, real-space STM
studies of Nbx Ta12x S2 and Tix Ta12x Se2 materials have provided a solid structural picture of the concepts of weak and
strong pinning in CDW materials.
B. Copper oxide superconductors
There have also been a number of SPM studies reported
in the literature that address the structure and electronic
properties of HTSC materials.2,6,20,21 Herein, we focus primarily on work from our laboratory that has addressed ~1!
the local crystal chemistry and doping, ~2! the role of structural defects in pinning magnetic flux lines, and ~3! the control of surface morphology by chemical etching.
We have performed extensive studies of the structural effects of metal substitution in the HTSC materials.6,11,21,23 A
particularly informative example of these studies is the case
of Pb-doped BSCCO where we have characterized the structural consequences of Pb substitution for Bi in both 2212 and
2201 crystal types. STM images of the Bi~Pb!–O layer of
Pb-doped BSCCO-2212 single crystals are shown in Fig. 3.22
Images of pure ~x50! BSCCO-2212 crystals exhibit a onedimensional, incommensurate superstructure that is characteristic of the BSCCO materials.11 As Pb is substituted for Bi
in these crystals, however, the superstructure exhibits increasing disorder. The one-dimensional superstructure is still
clear at low Pb concentrations ~3<0.2!, although there are
obvious distortions such as variations in the period from 25
to 39 Å. At higher Pb concentration we found that there was
little periodicity at length scales expected for the superstruc-
J. Vac. Sci. Technol. B, Vol. 14, No. 2, Mar/Apr 1996
1067
FIG. 4. The 60360 nm2 images of ~A! Bi2Sr2CuO6 , ~B! Pb0.15Bi1.85Sr2CuO6 ,
and ~C! Pb0.3Bi1.7Sr2CuO6 .
ture, and thus it is apparent that Pb-induced disorder ultimately destroys this one-dimensional superlattice. These
large structural changes also can have important implications
for superconductivity. In particular, we have shown that the
superconducting critical current density (J c ) increases significantly with Pb doping in BSCCO-2212.24 This increase in
J c is believed to arise from enhanced flux-line pinning by the
structural disorder caused by Pb substitution.
We have also carried out similar studies on Pb-substituted
BSCCO-2201 crystals. In general, the STM images of the
Pb-doped 2201 crystals were similar to those obtained on the
Pb-doped 2212 crystals; that is, increasing disorder was observed as the Pb concentration was increased ~see Fig. 4!. To
quantify the order in these crystals to compare our results to
those obtained by electron diffraction we have also calculated the two-dimensional structure factor S 2D~k!5ur~k!u2,
where r~k! is the Fourier transform of the atom density, from
images as a function of Pb concentration. Consideration of
this analysis suggests at least two significant points. First, at
1067
intermediate Pb concentrations our comparison of S 2D~k!
with in-plane electron diffraction patterns shows that the
multiple superlattices inferred from electron diffraction are
due to harmonics of the fundamental superlattice period.
Second, at higher Pb concentrations this same comparison
demonstrates that the absence of a superlattice in electron
diffraction patterns is due to the extreme disorder of the
structural modulation; that is, the crystal is not well ordered.
More generally, this approach of calculating S 2D~k! from
real-space images enables the quantitative determination of
local crystalline order, and thus will be complementary to
conventional diffraction techniques in elucidating microstructure of complex materials.
The surface defects described above as well as bulk defects can play an important role in pinning magnetic flux
lines in HTSC materials.25 Because flux-line pinning determines to a large extent the magnitude of J c in superconductors ~and therefore possible applications!, an understanding of
pinning is central to much work in the HTSC field. Several
techniques, including Bitter decoration and neutron diffraction, have been used to study the structure of flux-line lattices. While both of these techniques have provided significant insight into the structure of flux-line lattices, they have
not been able to provide direct information about defects that
pin the flux lines. STM and magnetic force microscopy are
capable of imaging both flux lines and crystal structure ~defects! simultaneously, and recent results suggest that these
promising techniques will play an increasingly important
role in the future.26,27
Alternatively, a new approach that we have developed
combining conventional Bitter decoration and AFM has
yielded significant insight into the structure and pinning of
magnetic flux lines.28 Bitter decoration provides a wellestablished technique for highlighting reproducibly large
numbers of flux lines and, when combined with AFM, enables us to image the decorated flux-line positions and map
out surface defects with nanometer scale resolution. Typical
AFM images obtained on BSCCO single crystals that had
been decorated in a magnetic field of 33 G are shown in Fig.
5. Two distinct types of structures are seen in these images:
the magnetic flux-line positions, which appear as small circular spots, and surface steps. Analysis of the surface topography demonstrates that these steps have heights of 30, 100,
and 300 nm, respectively. More important, we have shown
that the flux-line lattice structure can differ significantly depending on the surface step height.28
The flux-line lattice orientation is seen most clearly in
two-dimensional Fourier transform ~2DFT! power spectra of
selected regions of these images. In Fig. 5~A!, the flux-line
lattice orientation is independent of surface step as is clearly
evident when comparing the flux-line lattice reciprocal lattice vectors with the step direction. In Fig. 5~B! which contains a larger step, the 2DFT shows that a principle axis of
the flux-line lattice is aligned preferentially along the straight
section of the step. The overall orientation of the flux-line
lattice is not perturbed, however, by the curvature in the step
at the bottom of the image. The structure of the flux-line
1068
1068
FIG. 5. AFM images recorded on a BSCCO crystal that was decorated at 4.2
K in a magnetic field of 33 G. Each image shows a surface step that crosses
the crystal surface. The insets correspond to 2DFTs calculated from the
AFM images; the 2DFTs have been rotated by 90° so that the reciprocal
lattice vectors can be compared directly with the real-space directions. In
~A! and ~B! the 2DFTs were calculated from the entire images; in ~C! the
three 2DFTs were calculated from the areas of the image where they are
displayed. The images in ~A!, ~B!, and ~C! are 25.4, 31.5, and 49.6 mm on a
side, respectively. Reproduced from Ref. 28.
lattice in Fig. 5~C!, which contains the largest step, is more
complex and interesting. In the upper left of the image the
2DFT shows that the flux-line lattice orientation is pinned to
the step direction. The flux-line lattice in the upper right
portion of the image is also pinned in this same orientation.
However, the 2DFT demonstrates that the flux-line lattice
orientation in the lower right part of the image is rotated
relative to the upper part. This rotation follows the change in
step direction that occurs at the bottom of the image. Significantly, the two distinct flux-line lattice orientations lead to
the formation of a grain boundary in the lattice. We have
developed a model to understand the propagation of these
grain boundaries by considering the pinning energy associated with the surface step and the energy to elastically deform the flux lattice.25 More important, using the AFM data
has made it possible to demonstrate that the grain boundaries
propagate completely through the superconductor samples
and thus influence significantly the bulk structure of the fluxline lattice.
The complex surface properties of the HTSCs described
above have also led to difficulties in making reliable STM
spectroscopy measurements and in fabricating controlled
junctions needed for applications. These problems arising
from the complex surface properties of HTSC materials can
in principle be overcome by either ~1! a controlled removal
of specific oxide layers via etching,29 or ~2! through selective
termination of thin-film growth with a particular oxide
layer.30 We have recently begun systematic studies of the
FIG. 6. AFM images of a BSCCO crystal recorded ~A! before etching, ~B!
after etching with a dilute bromine solution, and ~C! after etching with a
concentrated bromine solution. The step heights in images ~B,C! are ;1.5
nm.
etching approach using force microscopy. The emphasis in
this work is to exploit specific chemical etchants to control
the surface morphology of these complex materials. For example, we have investigated the solvent and concentration
dependence of bromine/alcohol etching of BSCCO surfaces
and found that it is possible to vary the etching process from
layer by layer to nearly vertical ~see Fig. 6!. Figure 6~B!
shows an atomically flat BSCCO surface obtained by layerby-layer etching; the surface features in this image all correspond to half unit cell steps of ;1.5 nm. In contrast, Fig.
6~C! shows that higher etchant concentrations produce primarily vertical etching of the BSCCO surface. To demonstrate the generality of this technique we have also carried
out similar studies on La1.8Sr0.2CuO4 ~LSCO! crystals. The
step-terrace structure exhibited in Fig. 7 shows that it is pos-
J. Vac. Sci. Technol. B, Vol. 14, No. 2, Mar/Apr 1996
1069
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1
FIG. 7. AFM image of a LSCO single crystal surface etched with a bromine
solution. The step heights in this image are ;0.7 nm.
sible to obtain layer-by-layer etching with this system as
well. We believe that the ability to control the etching from
layer-by-layer to vertical will have significant implications in
fabrication steps such as planarization and pattern definition.
IV. CONCLUSIONS
In summary, we have reviewed SPM studies from our
laboratory that have focused on low-dimensional materials
exhibiting CDWs and HTSC. Temperature-dependent SPM
studies of the structural properties of CDW phases in metaldoped TaS2 and TaSe2 have provided a much deeper understanding of weak and strong CDW pinning, and have led to
the discovery of a hexatic CDW phase in niobium-doped
TaS2 . STM studies of metal-doped BSCCO 2212 and 2201
superconductors have been used to elucidate the complex
crystal chemistry of this system. These studies have provided
a clear explanation for enhanced critical currents in Pb-doped
materials, and highlight the general potential of local crystallography for unraveling complex materials problems. A new
approach combining AFM and decoration has also been developed and used to elucidate the interaction of magnetic
flux lines with surface defects, thus enabling the first quantitative assessment of pinning by surface roughness. Finally,
etching has been studied by in situ SPM and shown to be a
promising means for controlling the structure and electronic
properties of HTSC surfaces.

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